Monobloc compressor rotors, such as integrally-bladed rotors (IBRs) and one-piece impellers for gas turbine compressors may be formed by forging, heat treating and then machining. A certain amount of mechanical working of the material during forging is necessary to provide good mechanical properties, such as a high low cycle fatigue (LCF) life. Compressors must also be lightweight and maintain good mechanical properties at high temperature, and may have operational rotational speeds in excess of 30,000 rpm.
Once a mult has been forged into a blank and heat treated, the rotor blades are then machined from the forging. Alternately, as shown in U.S. Pat. No. 6,095,402, rather than machining the entirety of the blades, some axial-flow rotor designs permit the hub blade stubs to be machined from the blank, and then separately-fabricated airfoils are friction welded on to the stubs.
The rotor must also be attached to a shaft. Referring to FIGS. 2a and 2b, conventional possibilities for attachment of an impeller are to mechanically fasten the impeller / to a flanged shaft S with bolts B (FIG. 2a) or to provide a stub shaft SS integrally with impeller /, the stub shaft SS being threaded for connection to shaft S (FIG. 2b). Other options not shown are keyed shafts and friction fits. The approach of FIG. 2b is often preferred for airborne gas turbine applications to 2a, since the mechanical fasteners of FIG. 2a add unwanted weight, while keyed shafts add stress concentrations, and all designs present dynamics issues to the designer. The integral shaft impeller of FIG. 2b is provided by extruding a stub shaft SS during forging.
As described briefly above, in the forging process, large amounts of deformation are required to randomise the metallurgical texture and disrupt and remove the regions of weakness from the forging, to there by provide good mechanical properties such as LCF life. After mechanical working, heat treatment is performed. If the cross sectional area is beyond a limiting value, known as the “ruling section”, it is not possible to adequately heat treat and forge the centre of the forging, which reduces the mechanical properties of the alloy in that region. The somewhat trapezoidal shape of the impeller cross-section naturally makes this forging and heat treatment issue much more of a concern for the manufacture of impellers than it does for disc-type rotors. Also, the inclusion of an extruded shaft portion to the impeller greatly increases the cross sectional area of the part, exacerbating forging issues.
Although prior art forging and forging +extrusion processes work well with many titanium compressor rotor designs, a relatively new titanium alloy known as IMI834 (generally Ti-5.8Al, 4Sn, 3.5Zr, 0.7Nb), or Timetal 834, (a trade mark of Titanium Metals Corporation), presents new problems to the field of compressor rotor manufacture not adequately addressed by prior art technologies. IMI834 is known to have highly desirable properties for aircraft engine components, such as high temperature creep resistance, high tensile and fatigue strengths, and a relatively low density, however IMI834 also has several intrinsic material properties which make it a uniquely difficult material with which to work, especially in the provision of integral compressor rotors for gas turbines. Prior art manufacturing techniques result in a poor LCF life for gas turbine impellers, for example.
Therefore a need exists to address the problems of manufacturing compressor rotors, and particularly impellers, from IMI834 to provide improved LCF lives.